The present invention pertains to oil and gas production, and more particularly, to well completion.
After an oil or gas well is drilled and casing is installed, it is necessary to perforate the casing at the location of the oil or gas-bearing strata. Casing is perforated by either firing projectiles or by exploding a shaped charge. There are two main types of perforation devices; the tubing gun, which is lowered through the production tubing; and the casing gun, which is run into the well before the tubing is installed. The casing gun is larger and substantially more effective than the tubing gun, but the casing gun can be installed only when the tubing is not in place in the well.
When the well casing is perforated, it is often desirable to have the pressure within the well bore at a lower level than the pressure of the adjacent oil or gas-bearing strata so that the hydrocarbon products will flow rapidly into the well bore initially cleaning out the perforation holes and thence up to the surface due to the strata pressure alone. In many instances, the well bore is filled with drilling mud, salt water or other fluids which form a hydrostatic pressure head that inhibits the flow of hydrocarbons from the formation into the bore. The hydrostatic pressure may be so great as to force the fluid from the well into the formation thereby contaminating the area adjacent the well bore and inhibiting the flow of hydrocarbons. Therefore, in many cases, it is desirable to have a pressure differential toward the well bore to promote the flow of hydrocarbons into the well, especially at the time of perforating the well casing.
In U.S. Pat. No. 2,906,339 issued to Griffin, there is shown a method and apparatus for perforating well casing. This method shows the conventional technique of positioning a casing perforator by attaching the perforator to the lower end of the production tubing string and lowering the tubing string into the casing. When the perforator gun is in position, it is activated by dropping a weighted explosive charge through the tubing.
U.S. Pat. No. 3,706,344 to Vann likewise shows the method of positioning a casing perforator by joining the perforator to the end of the tubing string. After the tubing string is lowered into the casing to the predetermined depth, the perforator is activated.
In both of the above patents, the casing gun is positioned by manipulating the tubing string, an awkward and imprecise procedure. Therefore, there is a need for a method and related apparatus for easily and accurately positioning a casing gun perforator in a well while the production tubing is fixed in place and with a pressure differential toward the well bore.